1. Field of the Invention
The present invention concerns an agent for decontaminating contaminated metallic or cement-containing substances. The invention also concerns, however, a process for the production of this decontamination agent by using boric acid, which is contained in the primary cycles of pressure water reactors. The invention further concerns processes for using the decontamination agent. Although the decontamination agent in accordance with the invention is not restricted to the use of radioactively contaminated materials, the primary emphasis in the following description will be laid on this application.
2. Description of the Prior Art
In the past, the contaminated surface layers of reactor cooling conduits were frequently removed by means of aqueous mineral acid solutions. One such decontamination solution, with 20% nitric acid and 3% hydrofluoric acid, is cited, for example, in "Kernenergie", 11th year, 1968, page 285. Since, because of the aggressive nature of such mineral acid solutions, the removal process can only be controlled with great difficulty, there exists the danger that the pure metal below the contaminated surface layer will be corroded, so that weak points may arise, which may lead to the formation of leaks--which must in all cases be avoided. Of all the decontamination processes later developed in order to remove such or similar defects, the best known one must be the so-called "AP-Citrox" process ("Kernenergie", 11th year, 1988, page 285), in which the contaminated surface is first treated with an oxidizing alkaline permanganate solution to prepare for dissolution, and is then treated with a reducing, aqueous solution of dibasic ammonium citrate.
In U.S. Pat. No. 3,873,362, a similar two-stage decontamination process is described, in which, during the first stage, hydrogen peroxide is preferably used for oxidation, and, during the reducing, second process state, aqueous solutions of mixtures of mineral acids (sulfuric acid and/or nitric acid) and complex-forming substances, such as oxalic acid, citronellic acid, or formic acid, are employed.
In accordance with another known decontamination process taught in German Patent DE-PS No. 27 14 245, the contaminated metallic surface is treated with a cerous solution containing at least one cerium-IV-salt and a water-bearing solvent. A further decontamination process is described in European Patent Application, publication No. 00 73 366, in which an aqueous solution of formic acid and/or acetic acid is used as a decontamination agent, and, as a reducing agent, formaldehyde and/or acetaldehyde is used. In this process, it is particularly advantageous that a relatively slight need for chemicals exists, and, during the removal of the used decontamination solution, a quantity of precipitated radioactive substances corresponding approximately to the volume of the surface layers removed is used.
In the wet chemical decontamination processes which have been briefly described above, the basic concept is connected with the fact that the activity in the contaminated surface layer decreases with mass, as the surface layer itself is dissolved by the decontamination solution. The penetration depth of active material into the surface layer can be determined or measured before decontamination.
Decontamination tests on various metallic reactor components have only one conflict with the statement above, that the amount of residual activity is solely a function of the thickness of the surface layer removed. For various decontamination solutions, there are provided various decontamination factors with the same gravimetrically determined abrasion of layers. Research with a scanning electron microscope has shown that solid layers or islands of solids have formed on the decontaminated metal surfaces, in which active material is concentrated, and which are considered undesirable by-products of the specific abrasive reactions. Such variations are particularly observed in substances which contain silicon or aluminum, and thus in stainless steels and high-temperature materials, such as, for example, are used in helium-cooled high temperature reactors, and even in slightly alloyed steels. Apart from an undesirably high residual activity, the monitoring and control of the decontamination process is, because of the irregular removal of such surface layers, difficult, so that reliable decontamination is no longer ensured, and the previously stated corrosion damage has to be taken into account.
In the primary water cycle of pressurized water reactors, boric acid is found in concentrations of up to 3000 ppm. During the operation of such reactors, small quantities of the stated fluid precipitate as waste. This waste contains, in addition to boric acid, further contaminants, such as, for example, cobalt compounds, as well as solid contaminants, such as, for example, rust residues, materials fibers, dust, and the like. This waste can, in certain cases, be treated to such an extent that it is present in the form of a solid material.
The waste was previously generally concentrated to approximately 16 weight percent by means of evaporation, so that this concentrate then had an activity of 0.1 to 3 Ci/m.sup.3, and up to 1 g/l of solids (28,000 ppm boron). Such a concentrate may be solidified with cement (see also, for example, Nagra: Nationale Genossenschaft zur Lagerung radioaktiver Abfalle Technical Report, (84-09). A quantity of 123 kg concentrate solution/200 liter matrix, with a volumetric weight of 1.89 Mg/m.sup.3, that is, 123 kg (=114 liters with a density of 1.08 Mg/m.sup.3) is solidified in a matrix weighing 378 kg. The quantities of concentrate can amount to up to 10 m.sup.3 per nuclear plant per year. To remove this amount of concentrate, approximately 88 vessels were required, according to the above assumptions, whereby the volume of each vessel amounted to about 200 liters. With a price of 5,000.00 Swiss francs per vessel, including removal, the sum of 440,000.00 Swiss francs for the removal of the annually precipitating quantity of waste results.